1. Field of the Invention
The present invention related to a circuit for receiving optical signal.
2. Related Prior Art
A pre-amplifier with a type of trans-impedance amplifier (hereafter denoted as TIA) for receiving optical signal is requested to enhance the input dynamic range thereof. For instance, Japanese Patent Application published as JP-H10-335957A has disclosed a trans-impedance amplifier with a diode element connected in parallel with the feedback impedance. When the input current to the TIA becomes large, the diode connected in parallel with the feedback impedance turns on to prevent the output of the TIA from saturating.
Another Japanese Patent application published as JP-2003-163544 has a TIA with an FET connected in parallel with the feedback impedance as an element showing a variable resistance. The U.S. Pat. No. 5,801,588, has also disclosed such a configuration of the TIA. In the U.S. Pat. No. 7,418,213, the FET connected in parallel with the feedback impedance varies the impedance thereof so as to bypass the input current by responding the average of the input current that is obtained by filtering the input current, when the input current to the TIA becomes large.
For the TIA with a diode element in parallel with the feedback impedance, a Schottky diode would be necessary as a device having a low forward saturation characteristic to retain the low power supply condition of the TIA. The Schottky device is sometimes hard to be monolithically integrated with other circuit devices. The arrangement where the FET connected in parallel with the feedback impedance is necessary additional feedback loop to control the FET by the average of the input signal, which inevitably causes a cut-off in lower frequency regions. A capacitor with relatively large capacitance is necessary to lower the cut-off frequency. However, such a capacitor is hard to be monolithically integrated within the circuit, while, an external capacitor with such large capacitance is likely to increase a size of the circuit.